This invention concerns hypodermic syringes and particularly the safety thereof.
The safety of hypodermic syringes as addressed herein falls broadly into two categories, that is patient safety and practitioner safety, and these two matters will now be considered in turn.
When using a hypodermic syringe it is generally necessary for reasons of patient safety to check the position of the needle tip relative to blood vessels to ensure that potentially toxic material is not injected into the circulatory system. Intravascular injection gives rise to the risk of a toxic reaction, especially if administered quickly, when the injectant may travel around the body as a bolus to arrive at the brain in abnormally high concentration. This may happen with any injectant but is notably serious in the case of local anaesthesia, especially if the anaesthetic agent is combined with adrenaline, since adrenaline entering the circulation system may have fatal consequences. The danger is particularly severe in dental local anaesthesia because the injection site is so close to the patient's brain.
The standard method of checking the position of the needle tip relative to blood vessels is by a procedure known as aspiration. After the practitioner has inserted the needle, the plunger of the syringe is retracted by a small amount to obtain a sample of the surrounding body material. If the tip of the needle is in a blood vessel, a sample of blood will be drawn back into the syringe where it can be observed. Then, if blood is seen, the practitioner can begin again, altering the position of the needle. To allow for movement of the needle, aspiration may need to be repeated one or more times during the course of the injection.
There are two kinds of aspiration, active and passive. Active aspiration is a procedure in which the practitioner retracts the plunger manually. Active aspiration is somewhat awkward and may lead to movement of the needle, as well as of the plunger, making the aspiration uncertain. Passive aspiration (sometimes called “self-aspiration” which for the purposes of the present application is deemed to mean the same thing) is much safer. Passive aspiration is aspiration that takes place automatically when pressure on the plunger of the syringe is released, rather than the plunger having to be actively retracted. In other words, providing a syringe with means for passive aspiration means that the practitioner does not have to move the plunger manually to carry out the aspiration procedure. And this ensures that the injection site will be the same as the sampling site.
It is an object of the present invention to improve patient safety by providing a hypodermic syringe with passive aspiration.
The essential sharpness of hypodermic needles inevitably means that users occasionally injure themselves. Until comparatively recently, these so-called needlestick injuries were almost always no more than painful, and regarded as an occupational hazard by doctors, dentists and so forth. However perceptions have been changed by the spread of serious diseases such as auto-immune deficiency syndrome (AIDS) and hepatitis and the resurgence of tuberculosis in developed countries. Such diseases are readily transferred by needle and needlestick injuries are now perceived as potentially life-threatening. It will be understood that aspiration amplifies the threat by inducting a much greater amount of potentially infected material into the syringe.
The prevalence of needlestick injuries is indicated by a trade union estimate of twenty thousand such injuries a year in Scotland alone. The seriousness is reflected in data from UK occupational health departments showing eight hundred and thirteen reports of exposure to bloodborne viruses at work between July 1997 and June 2000. (And at least one practitioner caught HIV despite immediate prophylaxis). Official concern is marked by the Needlestick Safety and Prevention Act passed (unanimously) by the US Congress and signed into force in November 2000.
In dentistry it has been universal practice over many years to employ hypodermic syringes with needles that after use are resheathed, detached and placed in a container known as a sharps box for disposal. However the risk of a needlestick injury is present during the period between completing the injection and placing the needle in the sharps box. The risk is obviously greater if this period is extended—as when a practitioner puts a syringe aside after use (perhaps, understandably, to attend to the patient) instead of dealing with its disposal straight away. It is also, of course, not uncommon for a practitioner to compound the risk by passing the used syringe to a nurse or other assistant.
However the most acute, and avoidable, risk stems from post-injection disassembly, and in particular resheathing the needle. (Notably, regulations in USA prohibit resheathing of hypodermic needles but dentistry has a specific exemption). Disposability of the syringe as a whole will make it quick and easy for the practitioner to dispose of it immediately, and the needlestick risk will be substantially reduced. (The use of a disposable syringe in dentistry will also, of course, remove the need for the dental exemption).
It is therefore another object of the invention to improve practitioner safety by providing a hypodermic syringe which is disposable after use, that is as a wholes without the need to detach the needle or any other part.
At this point may be noted another aspect of patient safety which is particularly relevant to dental local anaesthesia. Dental surgeons invariably use a cartridge type of syringe which has a reusable actuating mechanism. A cartridge containing local anaesthetic agent—typically an amino-ester or an amino-amide mixed with a proportion of adrenaline—is loaded into a chamber of the actuating mechanism. The cartridge has a slideable bung arranged to be engaged by a plunger of the actuating mechanism when the cartridge is in the chamber. After the cartridge has been loaded into the chamber a needle is attached to its forward end, usually by means of a screw thread arrangement. Then the practitioner can depress the plunger to drive the bung into the cartridge and expel the contents thereof to make an injection in the usual way. After use, the cartridge and needle are disassembled and disposed of, and the actuating mechanism sterilised for reuse.
The problem in this, as will be appreciated by those knowledgeable in the science, is that certain disease vectors—such as the prion proteins associated with variant Creutzfeldt Jakob disease (VCJD)—can survive conventional sterilisation. It is therefore another object of the invention to provide a hypodermic syringe which does not require sterilisation after use.
A disposable syringe is the only effective safeguard against cross-contamination by way of the syringe, but the dental profession has not hitherto used disposable syringes for local anaesthesia. It will be appreciated, however, that a disposable syringe can go entirely into the sharps box waste container straight after use. And importantly, the dentist can do this using just the (one) hand holding the syringe, without exposing his/her other hand, any other part of his body or any ancillary (or even, as may be the case, another patient) to needlestick risk.
For the avoidance of doubt it should be observed that disposable syringes (which of course do not entail post-injection sterilisation) have been widely used over many years in medical and veterinary practice, but not in dentistry. By contrast, passive aspiration is well established in dentistry but not elsewhere. It is therefore a final object of the present invention to combine the benefits of disposability and passive aspiration to the advantage of all medical, dental and veterinary practice.
Certain previously known syringes will now be discussed.
In dental local anaesthesia the toxicity problem noted earlier has made pre-injection aspiration a standard recommendation—see, for instance, “Introduction to Dental Local Anaesthesia'” Mediglobe SA, 1990- and it was in this field that passive aspiration was first introduced, by Evers, more than thirty years ago. In U.S. Pat. No. 3,295,525 Evers et al disclose a hypodermic syringe of the cartridge type which is arranged to be automatically self-aspirating. The cartridge is provided with a bung having a bore extending through it to a resilient diaphragm at the head of the bung (that is, that part of the bung which engages the injectant in the cartridge). A specially designed nose on the end of the plunger in use extends through this bore to engage the diaphragm. When the plunger of the actuating mechanism is pressed the diaphragm is deflected inwardly and when the plunger is released the diaphragm automatically returns to its normal position, creating a pressure reduction within the cartridge to effect the desired aspiration. If aspiration is satisfactory the injection may proceed, the practitioner pressing the plunger further forward to cause a flange behind the nose to engage the bung and drive it into the cartridge and expel the injectant.
The Evers arrangement has a number of disadvantages. First, the requirement for a diaphragm complicates the manufacture of the bung. Second, the actuating mechanism has to be specially made or modified to provide the nose and the flange described above. Third, the special self-aspirating cartridge may only be used with this form of syringe. Fourth, the special syringe cannot be satisfactorily used with a standard cartridge because, without a bore in the bung, the extension may make the bung tip over and leak. And finally, the actuating mechanism is not disposable and, as noted earlier, its reuse requires sterilisation that is likely to be less than perfect.
In U.S. Pat. No. 4,216,771 Arlers et al disclose a hypodermic syringe with aspiration which is specifically stated to be a disposable article. However, this syringe does not provide passive aspiration. It has a plunger comprising circumferentially inner and outer parts connected together by a diaphragm at the head of the plunger. An aspiration rod is connected to the inner part and extends through a longitudinal channel in the plunger to an actuating member pivotally mounted at the outer end of the plunger. The practitioner effects aspiration by manually pivoting the actuating member to draw the inner part of the plunger backwards.
Aside from its complexity, with a number and variety of moving parts, the Arlers syringe offers active aspiration rather than passive aspiration. It therefore fails to secure the benefits of passive aspiration noted hereinbefore. In fact U.S. Pat. No. 4,216,771 points away from passive aspiration, criticising one previous arrangement (apparently the Evers syringe) as being difficult and awkward to use and another as being complicated and expensive. The present invention enables both these criticisms to be obviated.
An alternative to Evers's arrangement for passive aspiration, and more recent, is shown in U.S. Pat. No. 5,411,488 to Pagay et al. This discloses a syringe barrel pre-filled with injectant and closed by a slideable bung (or “plunger”) of elastomeric material. The barrel has means at one end for attachment of a needle before making an injection. A plunger rod has a convex tip which fits into a blind bore in the bung and before use is connected thereto by a collar captive on the tip and formed with a screw thread for connection to the bung. The collar has limited sliding movement with respect to the plunger rod so that when the latter is depressed it first of all moves forward against the end of the blind bore to distort the head of the bung inwardly. If pressure on the plunger rod is then released the head of the bung returns to its normal shape, thereby effecting aspiration. Subject to satisfactory aspiration the plunger rod may then be depressed further, when a shoulder on the rod engages the collar screwed into the bung to drive the bung into the barrel and eject its contents.
Those skilled in the science will appreciate that Evers and Pagay provide aspiration in substantially the same way: in Pagay the convex tip of the plunger deforms the head of the bung just as in Evers the extended nose the plunger acts on the elastic diaphragm of the bung. It follows that Pagay's arrangement shares a number of the disadvantages noted above in relation to Evers. The provision of a screw thread in Pagay makes the bung even more complex. Similarly Pagay's plunger is complicated, and it requires the practitioner to connect it to the bung before an injection can be made. There is no mention of disposability in U.S. Pat. No. 5,411,488, but the very complexity of the plunger shown therein (and by inference its cost) suggests that practitioners would be encouraged to reuse it; and the persistence of certain disease vectors noted earlier makes this hazardous even with sterilisation. Significantly, thirty years elapsed between Evers and Pagay but there was no move towards combining disposability with passive aspiration in all that time, nor has there been in the years which have elapsed since Pagay.